Motor vehicle bumpers have an outer skin which serves firstly to provide an esthetic function of extending the bottom of the front or back portion of the bodywork of the vehicle, and secondly a mechanical function of constituting an impact surface for receiving impacts to which the vehicle may be subject.
Behind the bumper skin, there is provided a shock absorber whose function is to take up the forces transmitted by the skin and to absorb the major fraction of the energy thereof before transferring it to the side rails of the vehicle.
In general, the shock absorbers are situated in line with the side rails and they are interconnected by a cross-member which transfers to them the forces to which the bumper is subject. The shock absorbers are made to measure and are quite expensive, such that in certain down-market vehicles, they are replaced by a low energy shock absorbing system of the polypropylene foam type whose sole purpose is to limit damage to the skin of the bumper in the event of small bumps.